Buzz saw noise was discovered by aviation industry in the 1960's when the high bypass ratio fan engine was first used to power the jumbo jet aircraft. Buzz saw noise was generated when the high bypass ratio fan engine was run at take-off and max-cruise powers. The noise propagated through the engine inlet sounded like that of a buzz saw hard at work in the woods falling timbers. Noise engineers and technicians, the present inventor was one of them, coined this fluttering noise the buzz saw noise.
Paris air show would be the occasion and place to display the brand new jumbo jet 747 and that was the intention of the Boeing management. The new 747 jumbo jet engine JT9D, however, was experiencing fan stall and engine surge at high power settings; whether it was safe to fly over the ocean to Paris was the concern. Boeing management would like to witness in person how the engine operates before signing off the maiden flight trip; in doing so, they heard this funny engine noise at take-off power as if the fan blades are about to fly apart. “We need to fix It.”, was the comment. Noise research engineers asked the technicians on the test site. “what does this funny noise sounds like? “Sort of like a buzz saw.”
From ⅓-octave band noise spectrum analyses, the funny noise spectrum displayed high amplitude low frequency broadband noise, which was absent for low engine power settings. It was not evident that this noise was from the engine inlet, and it could be that the jet noise spilled over into the front hemisphere. But jet noise does not have this funny flutter characteristic.
Subsequent digital narrow band spectral analyses using the newly developed FFT (fast Fourier transform) technique render that the spectrum content of this funny engine noise consists of a multitude of tones at multiple of fan shaft frequency; and hence an alternative name of this funny engine noise was multiple pure tone noise.
At high power settings, the fan rotor blade tip of the high by-pass ratio fan engine rotates at transonic speeds. Aerodynamically, at this transonic speed, a fan blade leading edge shock system emerges, and this funny engine noise could be associated with this shock system. Shock is essentially a non-linear phenomenon, linear acoustic treatment of the shock associated noise is an over simplification.
Buzz saw noise control problem is a tall order and is not to be considered a simple task. Instead of a single blade passing frequency tone, there are many (say, 20 or 30) buzz saw tones waiting to be attenuated. The daunting task of controlling a multitude of buzz saw tones often precipitates a tendency of circumventing the control problem toward a psycho-acoustic opaque impression that these low frequency tones might be less troubling. Apparently, existing technique of inlet peripheral lining application is not adequate to deal with so many buzz saw tones. Adaptive control, wave cancellation techniques to deal with so many tones appear also hopeless.
Tangential blowing, otherwise known as wall jet, is a branch of viscous aerodynamics, which serves to inject a layer of grazing flow along a surface to control the complications of flows over the surface, such as excessive boundary layer development or flow separation.
In addition to the classical boundary layer flow control, we aim to introduce tangential blowing to improve buss saw noise attenuation performance of the buzz saw lining for community noise control and air craft cabin noise reduction.
Relevant technology landscape of the inlet invention includes the following areas:                Buzz saw lining,        Tangential blowing,        Quiet air supply system,        Parasitic recirculation flow from fan bleed or compressor bleed,        Inlet and nacelle for minimum cruise drag and reduction of trip fuel consumption,        Inlet/engine integration technology,        Fan tip flow energization,        Cabin noise control at cruise condition.The references at the end of this disclosure show some of the recent works by the aviation industry.        